A system for in situ replacement of a drill bit for a ground drill is described in Applicant's International Application No PCT/AU94/00322 (WO 94/29567), the contents of which is incorporated herein by way of reference. The system in WO 94/129567 comprises a drive sub which is adapted for connection to a lower end of a core barrel attached to a drill pipe; a core drill bit composed of a plurality of separate bit segments; a bit locking sleeve for selectively locking the bit segments onto seats provided about an inner surface of the drive sub and subsequently releasing the bit segments from those seats; and, a down hole tool for operating the bit locking sleeve and installing and retrieving the drill bit segments.
A slidable cradle extends from a lower end of the down hole tool for carrying the bit segments to and from the drive sub. When installing the bit segments, the cradle is extended from a lower end of the tool against the bias of a spring. Bit segments are then held by elastic bands about the cradle with one end abutting a stop provided at a distant end of the cradle and an opposite end of each segment bearing against a head of the down hole tool. When the down hole tool is lowered into the ground drill and reaches a predetermined position within the drive sub, the cradle is retracted into the main body of the tool by the spring. This causes an upper end of the bit segments to slide along the head of the tool so as to extend laterally of the periphery of the tool. The bit locking sleeve is simultaneously pushed by the tool so as to catch the ends of, and move inside, the bit segments thereby expanding the bit segments to the inner diameter of the drive sub and locking the bit segments into a cutting position.
When it is necessary to change the core bit and thus retrieve the bit segments, the tool is again lowered into the drill pipe and drive sub with the cradle locked into the extended position. At a predetermined position, the down hole tool engages the bit locking sleeve at which time the cradle extends from the lower end of the drive sub. The down hole tool is then pulled upwards a short distance. This pulls the bit locking sleeve upwards thereby releasing the bit segments from the seats on the drive sub. The bit segments collapse onto the cradle by action of the elastic band. Upon further upward pulling of the down hole tool, the tool releases itself from the bit locking sleeve and can thus be pulled to the surface with the bit segments.
Field trials of the above system have proved very successful. However, it is believed that one possible source of failure of the system is that when the bit segments are released so as to collapse onto the cradle there is a remote but nevertheless existent possibility that they can jam in the drive sub and in particular with the bit locking sleeve. This can arise because when the bit segments are released and commence to collapse onto the cradle, they pivot or tilt radially inwardly from the lower end of the bit segments with increasing angle until the bit locking sleeve is pulled completely away from the bit segment. If the tool is withdrawn very quickly so that the uppermost end of the bit segments have not had sufficient time to fully collapse onto the cradle, perhaps due to the viscous nature of the fluid within which the drill is operating, the upper end of the bit segments can jam with the lower end of the bit locking sleeve.